Re: warning horn test for the temp sensors
I have an I/O, but I am going to take a 'stab' at this....
Most temperature switch horns can be tested by simply grounding the switch's lead to the engine block, with the key in the 'on' position.
As for the switch itself, I have never seen a test that didn't require putting it into hot water on the stove top, and reading the electrical resistance as the water reached the switch's trigger temperature. I have always assumed that it would be easier to replace any suspect switch, than go through all of those 'hoops'!
Happy boating!